1. Field of the Invention
The present invention relates to a variable-reluctance (VR) resolver used for, for example, measurement or control of rotational angle or position, and to a rotational angle sensor using the same.
2. Description of the Related Art
A variable-reluctance (VR) resolver, which includes a stator having an excitation winding and output coils wound around its magnetic poles, and a rotor having an arbitrary salient pole shape, outputs a two-phase signal including a SIN signal voltage and a COS signal voltage, which vary with the rotational angle of the rotor. Such a VR resolver must output a resolver signal whose shaft angle multiplier is 1× and which serves as a reference for detection of an absolute position. In the case where the stator and the rotor are assembled in a misaligned state; i.e., the center axis of the rotor is deviated from the center axis of the stator, output signal voltages, which vary with the rotational angle of the rotor, greatly deviate from the designed output signal voltages, whereby the accuracy of the resolver deteriorates considerably.
The accuracy deterioration occurs because of the following reason. In the case where the shaft angle multiplier of a resolver is 1×, the shape of the salient pole is determined to have a single peak within a single rotation (mechanical angle: 360 degrees) of an input rotary shaft. Therefore, the change in radius of the salient pole per unit rotational angle becomes small, and thus, the amounts of change in the output signal voltages per unit rotational angle become small. Accordingly, even a small center deviation between the stator and the rotor produces large errors in the output signal voltages.
Conventionally, an absolute-position detection apparatus which can solve the above-described problem has been proposed (see, for example, Japanese Patent Application Laid-Open (kokai) No. H03-148014). The absolute-position detection apparatus utilizes, in combination, a resolver whose shaft angle multiplier is 1× and in which the phase of a detection signal changes by 360 degrees when the rotary shaft rotates one turn (hereinafter referred to as “1× resolver”) and a resolver whose shaft angle multiplier is n× and in which the phase of a detection signal changes by 360 degrees every time the rotary shaft rotates a 1/n turn (hereinafter referred to as “n× resolver”). In the apparatus, the 1× resolver detects a pole corresponding to the resolution (1/n turn), and the rotational angle position within the detected pole (an area corresponding to 1/n turn) is calculated on the basis of the detection signal from the n× resolver.
The term “shaft angle multiplier” refers to the ratio of an output electrical angle θe of a resolver to an actual input mechanical angle θm of the resolver, and in general, the mechanical angle θm is obtained through division of the output electrical angle θe by the shaft angle multiplier.
FIG. 5 is a block diagram of a conventional double-speed rotation detector which uses two resolvers.
FIG. 6 shows output characteristics of the conventional double-speed rotation detector of FIG. 5.
An input shaft 201, which is coupled to an object whose rotation is to be detected, is connected directly to an n× resolver 202 and indirectly to a 1× resolver 204, via a speed reducer 203 having a speed reduction ratio of 1/n. An output signal from the n× resolver 202 is passed through a synchronous detector 205 so as to remove an excitation frequency component from the output signal, and then converted to a digital signal by means of a resolver-digital (R/D) converter 207. The thus-obtained digital signal is input to a synthesizing circuit 209. Similarly, an output signal from the 1× resolver 204 is passed through a synchronous detector 206 so as to remove an excitation frequency component from the output signal, and then converted to a digital signal by means of a resolver-digital (R/D) converter 208. The thus-obtained digital signal is input to the synthesizing circuit 209.
The R/D converter 207 repeatedly outputs the same signal (a sawtooth signal which continues over an electrical angle of 360°) n times during a single turn (mechanical angle: 360°) of the input shaft.
The R/D converter 208 repeatedly outputs the same signal (a sawtooth signal which continues over an electrical angle of 360°) a single time during a single turn (mechanical angle: 360°) of the input shaft.
When sine-wave and cosine-wave output signals of each of the n× resolver 202 and the 1× resolver 204, which have a 90° phase difference therebetween, are represented by Va and Vb, and the rotational angle is represented by θ, the output signals Va and Vb are sine wave and cosine wave signals which undergo amplitude modulation in accordance with the rotational angle θ. However, since instantaneous values of the output signal voltages Va and Vb cannot be used as they are for calculation, the output signals Va and Vb undergo synchronous detection at the synchronous detectors 205 and 206, respectively, so as to remove the excitation frequency component therefrom. The thus-obtained signal voltages are converted to digital signals by means of the R/D converters 207 and 208.
On the basis of the digital signal output from the R/D converter 207, the synthesizing circuit 209 produces n triangular wave segments for a single period (mechanical angle: 360°) as shown in section (b) of FIG. 6, wherein each wave segment continues over a period of 2π/n. The period of 2π/n corresponding to an electrical angle of 360° is represented by a serial number, which serves as an identifier for the poles or wave segments. The maximum value of the triangular wave segment is 2π (rad: electrical angle).
Further, on the basis of the digital signal output from the R/D converter 208, the synthesizing circuit 209 produces a single triangular wave segment for a single period (mechanical angle: 360°) as shown in section (a) of FIG. 6, wherein the wave segment continues over a period of 2π (rad). The maximum value of the triangular wave segment is 2π (rad: electrical angle).
The characteristic charts of FIGS. 6(a) and 6(b) show the relation between a point on the characteristic chart for the 1× resolver and a corresponding pole on the characteristic chart for the n× resolver.
Even in the conventional apparatus which uses a 1× resolver and an n× resolver in combination, a detection signal output from the 1× resolver is still used as a reference. When a characteristic curve of a 1× digital signal obtained through R/D conversion of the output voltage signal of the 1× resolver is drawn, as shown in FIG. 6(a), its slope is small. This means that a small variation in input causes a large variation in output, so that output errors are easily generated. Moreover, when the shaft angle multiplier n of the n× resolver is increased (the number of salient poles is increased), a correct pole on the characteristic chart for the n× resolver may fail to be selected if the output of the 1× resolver does not change linearly, because of influence of errors. Therefore, the conventional apparatus cannot solve the problem involved in the conventional 1× resolver such that the detection signal is very likely to be influenced by errors.
In addition, since both a 1× resolver and an n× resolver are incorporated, the size of the apparatus increases. Moreover, since the resolvers are connected together via a speed reduction mechanism, operational malfunctions occur as a result of mechanical vibration, noise, and wear.